Bicycle Road Rally   I got this idea from an adults car road rally we have in our neighborhood each year that leads to a dinner and cash prizes.  You develop a puzzle that leads you on a trail in your neighborhood to eventually uncover a prize.  You develop clues.  Your birthday group travels on their bikes to their first clue.  Example:  Go to the corner of the street you live on.  What is the third letter in the street name that crosses yours? Fill in the first blank with that letter on your puzzle. Send them to other destinations.  What is the color of the front door of the neighborhood clubhouse?  Fill in the first letter of the color in your puzzle for the second blank.  Clue No. 1 fills in the first blank of your puzzle, clue no. 2 the second, and so on until you create the name of a place to find a prize.  And that tells them where to go to find the prize.  When they get to the destination, there could be more clues to find the prize.  Example: Look in the place that gets really hot!  (Barbecue grill). We had gift certificates to the Sweet Factory as our prize. They were wrapped up in pretty boxes with some candy included. Then we all biked back to our house, and had cake and presents.  This was a party for 10 year old girls.  You have to ask your guests to bring their bikes and helmets. I wouldn't try this with kids younger than 8 years old, just because it might be a safety issue.